1. Field of the Invention
The present invention relates in general to an electronic ballast for a high-intensity discharge (HID) lamp and, more particularly, to an electronic ballast for the HID lamp of a high wattage.
2. Description of the Prior Art
The HID lamp such as a sodium (natrium) vapor lamp, a mercury lamp, a metal halide lamp and the like needs a restart time in its characteristic. A prior typical magnetic ballast for a discharge lamp continuously tries a start operation until the lamp is relighted. Since the magnetic ballast is made of a coil and an iron core, the coil and the iron core seldom have a damage during the relight of the lamp.
In an electronic ballast, however, if a trial to start the discharge lamp is continued till a light-on of the discharge lamp, devices such as semiconductor devices of the electronic ballast may be damaged, which may result in a breakdown of the electronic ballast. In order to avoid the damage, the electronic ballast has in general a time delay circuit so as to adjust a relight time of the discharge lamp.
Meanwhile, since a high voltage should be applied in order to drive the HID lamp of the high wattage, a secondary voltage higher than a conventional voltage should be made. Besides, it is inevitable to flow a large amount of current into a switching device of the electronic ballast because the HID lamp of the high wattage needs a large lamp current. Furthermore, as a load generates a high voltage, the semiconductor switching device should have a high withstand voltage and a high rating current. As for now, a bipolar transistor and a metal-on-silicon field effect transistor (MOSFET) can meet the above requirement and thus they have been used as a switching device.
The bipolar transistor with the high withstand voltage and the high rating current has a low amplification factor so that a conventional base driving circuit can drive a HID lamp of a low wattage but can not drive the HID lamp of the high wattage. So, most electronic ballast circuits for the HID lamp of the high wattage uses the MOSFET rather than the bipolar transistor. But, the MOSFET device is capable of supplying a driving power about 200 watts at most, thus it can be said that the MOSFET is not available for the HID lamp of the high wattage, for example, 400 watts.
Meanwhile, the bipolar transistor has higher withstand voltage and allowable current than the MOSFET so that the bipolar transistor has an advantage in a large current drive. Accordingly, in order to drive the HID lamp of the high wattage, the bipolar transistor has to be employed as the switching device.